The present invention relates to improved techniques for catalytic reforming, particularly, catalytic reforming under low-sulfur, and low-sulfur and low-water conditions. More specifically, the invention relates to the discovery and control of problems particularly acute with low-sulfur, and low-sulfur and low-water reforming processes.
Catalytic reforming is well known in the petroleum industry and involves the treatment of naphtha fractions to improve octane rating by the production of aromatics. The more important hydrocarbon reactions which occur during the reforming operation include the dehydrogenation of cyclohexanes to aromatics, dehydroisomerization of alkylcyclopentanes to aromatics, and dehydrocyclization of acyclic hydrocarbons to aromatics. A number of other reactions also occur, including the dealkylation of alkylbenzenes, isomerization of paraffins, and hydrocracking reactions which produce light gaseous hydrocarbons, e.g., methane, ethane, propane and butane. It is important to minimize hydrocracking reactions during reforming as they decrease the yield of gasoline boiling products and hydrogen.
Because there is a demand for high octane gasoline, extensive research has been devoted to the development of improved reforming catalysts and catalytic reforming processes. Catalysts for successful reforming processes must possess good selectivity. That is, they should be effective for producing high yields of liquid products in the gasoline boiling range containing large concentrations of high octane number aromatic hydrocarbons. Likewise, there should be a low yield of light gaseous hydrocarbons. The catalysts should possess good activity to minimize excessively high temperatures for producing a certain quality of products. It is also necessary for the catalysts to either possess good stability in order that the activity and selectivity characteristics can be retained during prolonged periods of operation; or be sufficiently regenerable to allow frequent regeneration without loss of performance.
Catalytic reforming is also an important process for the chemical industry. There is an increasingly larger demand for aromatic hydrocarbons for use in the manufacture of various chemical products such as synthetic fibers, insecticides, adhesives, detergents, plastics, synthetic rubbers, pharmaceutical products, high octane gasoline, perfumes, drying oils, ion-exchange resins, and various other products well known to those skilled in the art.
An important technological advance in catalytic reforming has recently emerged which involves the use of large-pore zeolite catalysts. These catalysts are further characterized by the presence of an alkali or alkaline earth metal and are charged with one or more Group VIII metals. This type of catalyst has been found to advantageously provide higher selectivity and longer catalytic life than those previously used.
Having discovered selective catalysts with acceptable cycle lives, successful commercialization seemed inevitable. Unfortunately, it was subsequently discovered that the highly selective, large pore zeolite catalysts containing a Group VIII metal were unusually susceptible to sulfur poisoning. See U.S. Pat. No. 4,456,527. Ultimately, it was found that to effectively address this problem, sulfur in the hydrocarbon feed should be at ultra-low levels, preferably less than 100 parts per billion (ppb), more preferably less than 50 ppb to achieve an acceptable stability and activity level for the catalysts.
After recognizing the sulfur sensitivity associated with these new catalysts and determining the necessary and acceptable levels of process sulfur, successful commercialization reappeared on the horizon; only to vanish with the emergence of another associated problem. It was found that certain large pore zeolite catalysts are also adversely sensitive to the presence of water under typical reaction conditions. Particularly, water was found to greatly accelerate the rate of catalyst deactivation.
Water sensitivity was found to be a serious drawback which was difficult to effectively address. Water is produced at the beginning of each process cycle when the catalyst is reduced with hydrogen. And, water can be produced during process upsets when water leaks into the reformer feed, or when the feed becomes contaminated with an oxygen-containing compound. Eventually, technologies were also developed to protect the catalysts from water.
Again commercialization seemed practical with the development of various low-sulfur, low-water systems for catalytic reforming using highly selective large-pore zeolite catalysts with long catalytic lives. While low-sulfur/low-water systems were initially effective, it was discovered that a shut down of the reactor system can be necessary after only a matter of weeks. The reactor system of one test plant had regularly become plugged after only such brief operating periods. The plugs were found to be those associated with coking. However, although coking within catalyst particles is a common problem in hydrocarbon processing, the extent and rate of coke plug formation exterior to the catalyst particles associated with this particular system far exceeded any expectation.
Accordingly, one object of the invention is to provide a method for reforming hydrocarbons under conditions of low sulfur which avoids the aforementioned problems found to be associated with low-sulfur processes, such as brief operating periods.
It is another object of the invention to provide a reactor system for reforming hydrocarbons under conditions of low sulfur which permits longer operating periods.
After a detailed analysis and investigation of the coke plugs of low-sulfur reactor systems, it was surprisingly found that they contained particles and droplets of metal; the droplets ranging in size of up to a few microns. This observation led to the startling realization that there are new, profoundly serious, problems which were not of concern with conventional reforming techniques where process sulfur and water levels were significantly higher. More particularly, it was discovered that problems existed which threatened the effective and economic operability of the systems, and the physical integrity of the equipment as well. It was also discovered that these problems emerged due to the low-sulfur conditions, and to some extent, the low levels of water.
For the last forty years, catalytic reforming reactor systems have been constructed of ordinary mild steel (e.g., 2xc2xc Cr 1 Mo). Over time, experience has shown that the systems can operate successfully for about twenty years without significant loss of physical strength. However, the discovery of the metal particles and droplets in the coke plugs eventually lead to an investigation of the physical characteristics of the reactor system. Quite surprisingly, conditions were discovered which are symptomatic of a potentially severe physical degradation of the entire reactor system, including the furnace tubes, piping, reactor walls and other environments such as catalysts that contain iron and metal screens in the reactors. Ultimately, it was discovered that this problem is associated with the excessive carburization of the steel which causes an embrittlement of the steel due to injection of process carbon into the metal. Conceivably, a catastrophic physical failure of the reactor system could result.
With conventional reforming techniques carburization simply was not a problem or concern; nor was it expected to be in contemporary low-sulfur/low-water systems. And, it was assumed that conventional process equipment could be used. Apparently, however, the sulfur present in conventional systems effectively inhibits carburization. Somehow in conventional processes the process sulfur interferes with the carburization reaction. But with extremely low-sulfur systems, this inherent protection no longer exists.
FIG. 1A is a photomicrograph of a portion of the inside (process side) of a mild steel furnace tube from a commercial reformer. The tube had been exposed to conventional reforming conditions for about 19 years. This photograph shows that the surface of the tube has remained essentially unaltered with the texture of the tube remaining normal after long exposure to hydrocarbons at high temperatures (the black portion of the photograph is background).
FIG. 1B is a photomicrograph of a portion of a mild steel coupon sample which was placed inside a reactor of a low-sulfur/low-water demonstration plant for only 13 weeks. The photograph shows the eroded surface of the sample (contrasted against a black background) from which metal dusting has occurred. The dark grey-like veins indicate the environmental carburization of the steel, which was carburized and embrittled more than 1 mm in depth.
Of course, the problems associated with carburization only begin with carburization of the physical system. The carburization of the steel walls leads to xe2x80x9cmetal dustingxe2x80x9d; a release of catalytically active particles and melt droplets of metal due to erosion of the metal.
The active metal particulates provide additional sites for coke formation in the system. While catalyst deactivation from coking is generally a problem which must be addressed in reforming, this new significant source of coke formation leads to a new problem of coke plugs which excessively aggravates the problem. In fact, it was found that the mobile active metal particulates and coke particles metastasize coking generally throughout the system. The active metal particulates actually induce coke formation on themselves and anywhere that the particles accumulate in the system resulting in coke plugs and hot regions of exothermic demethanation reactions. As a result, an unmanageable and premature coke-plugging of the reactor system occurs which can lead to a system shut-down within weeks of start-up. Use of the process and reactor system of the present invention, however, overcomes these problems.
Therefore, a first aspect of the invention relates to a method for reforming hydrocarbons comprising contacting the hydrocarbons with a reforming catalyst, preferably a large-pore zeolite catalyst including an alkali or alkaline earth metal and charged with one or more Group VIII metals, in a reactor system having a resistance to carburization and metal dusting which is an improvement over conventional mild steel reactor systems under conditions of low sulfur and often low sulfur and low water, and upon reforming the resistance being such that embrittlement from carburization will be less than about 2.5 mm/year, preferably less than 1.5 mm/year, more preferably less than 1 mm/year, and most preferably less than 0.1 mm/year. Preventing embrittlement to such an extent will significantly reduce metal dusting and coking in the reactor system, and permits operation for longer periods of time.
And, another aspect of the invention relates to a reactor system including means for providing a resistance to carburization and metal dusting which is an improvement over conventional mild steel systems in a method for reforming hydrocarbons using a reforming catalyst such as a large-pore zeolite catalyst including an alkaline earth metal and charged with one or more Group VIII metals under conditions of low sulfur, the resistance being such that embrittlement will be less than about 2.5 mm/year, preferably less than 1.5 mm/year, more preferably less than 1 mm/year, and most preferably less than 0.1 mm/year.
Thus, among other factors, the present invention is based on the discovery that in low-sulfur, and low-sulfur and low-water reforming processes there exist significant carburization, metal dusting and coking problems, which problems do not exist to any significant extent in conventional reforming processes where higher levels of sulfur are present.
This discovery has led to intensive work and development of solutions to the problems, which solutions are novel to low-sulfur reforming and are directed to the identification and selection of resistant materials for low-sulfur reforming systems, ways to effectively utilize and apply the resistant materials, additives (other than sulfur) for reducing carburization, metal dusting and coking, various process modifications and configurations, and combinations thereof, which effectively address the problems.
More particularly, the discovery has led to the search for, identification of, and selection of resistant materials for low-sulfur reforming systems, preferably the reactor walls, furnace tubes and screens thereof, which were previously unnecessary in conventional reforming systems such as certain alloy and stainless steels, aluminized and chromized materials, and certain ceramic materials. Also, it was discovered that other specific materials, applied as a plating, cladding, paint, etc., can be effectively resistant. These materials include copper, tin, arsenic, antimony, germanium, brass, lead, bismuth, chromium, intermetallic compounds thereof, and alloys thereof, as well as silica and silicon based coatings. In one preferred embodiment of the invention there is provided a novel and resistant tin-containing paint.
Furthermore, the discovery led to the development of certain additives, hereinafter referred to as anticarburizing and anticoking agents, which out of necessity are essentially sulfur free, preferably completely sulfur free, which are novel to reforming. Such additives include organo-tin compounds, organo-antimony compounds, organo-bismuth compounds, organo-arsenic compounds and organo-lead compounds.
Also, the problems associated with low-sulfur reforming has led to the development of certain process modifications and configurations previously unnecessary in conventional reforming. These include certain temperature control techniques, the use of superheated hydrogen between reactors, more frequent catalyst regenerations, the use of staged heaters and tubes, the use of staged temperature zones, the use of superheated raw materials, and the use of larger tube diameters and/or higher tube velocities.